1. Field of the Invention
The present invention generally relates to analog-to-digital (A/D) converters and, more particularly, to a single chip A/D converter with built-in test circuitry that provides code-density histogram data to an external system.
2. Description of the Prior Art
A/D converters, including so-called flash A/D converters, are currently tested by first placing the A/D converter in a test bed wherein the electrical noise is negligible in comparison to the resolution of the A/D converter under test. The A/D converter analog input is simulated with a sinusoidal wave signal of sufficient spectral purity that the total distortion of the sinusoidal wave signal is negligible in comparison to the resolution of the A/D converter. The A/D converter digital output codes are then read into a random access memory (RAM) as fast as the A/D converter produces them. The resulting data is then analyzed by a digital computer to yield (1) missing output codes, (2) integral non-linearity, and (3) differential non-linearity.
There are two chief problems with this approach. As the A/D converter speeds become faster and A/D converter resolutions become finer, it becomes increasingly difficult to find RAM which will read A/D converter output codes at speed without adding an intolerable amount of electrical noise to the electrical noise floor of the test bed. In addition, the A/D converter characterization cannot easily be done later when the A/D converter has been incorporated into a larger system.
What is needed is a self-characterizing A/D converter which is testable without the usual requirement of an external system which is capable of acquiring data at the speed of operation of the A/D converter under test.